It is customary to formulate herbicides as water-dispersable compositions which can be mixed readily with water and applied by means of spraying apparatus. An important class of herbicides which can be applied by this means comprises the asymmetrical N-amino triazines, of which 4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one (metribuzin) is used widely. Agriculturally useful formulations of this herbicide are marketed throughout the world under the trademark Lexone.RTM. weed-killing compounds (trademark of E. I. du Pont de Nemours and Company, Wilmington, Del.) or Sencor.RTM. (trademark of Mobay Chemical Company, Kansas City, Mo.).
Metribuzin is employed commonly for selective control of broadleaf weeds in crops such as soybeans. To improve the spectrum of weed control, commercially available formulations of metribuzin (such as Lexone.RTM. 50WP, Lexone.RTM. 4L, Lexone.RTM. 75DF, Sencor.RTM. 50WP and Sencor.RTM. 4F ) are combined commonly in spray tanks with grass-killing herbicides. The grass-killing herbicides often are formulated as aromatic hydrocarbon-based emulsifiable concentrates. Representative examples of grass-killing herbicides which can be combined with metribuzin are Treflan.RTM. (trademark of Elanco Products Company, Indianapolis, Ind., for a trifluralin formulation), Prowl.RTM. (trademark of American Cyanamid Co., Bound Brook, N.J., for a penoxalin formulation), Basalin.RTM. (trademark of BASF Wyandotte Corp., Parsippany, N.J., for a fluchloralin formulation), and Tolban.RTM. (trademark of Ciba-Geigy Corp., Greensboro, N.C., for a profluralin formulation). When applied in the field, these mixtures are diluted in spraytanks with water or liquid fertilizer to form spray mixtures.
Suspensions of metribuzin in water can be utilized without difficulty. When a solution of grass herbicide in an aromatic hydrocarbon solvent is added to the metribuzin/water suspension, however, significant growth and settling of metribuzin crystals can occur. Variation of the grass herbicide and its associated aromatic hydrocarbon solvent results in differing rates and extent of crystallization with the greatest problem observed when the solubility of metribuzin in the hydrocarbon solvent is high. This crystal growth is observed especially in those instances when the spray mixture is allowed to stand unagitated for periods of twelve hours or more. Such situations are not uncommon, occurring, for example, when rain or darkness delays completion of spraying in the field.
The crystallization of metribuzin within the spray apparatus can result in the plugging of field-sprayer spray nozzles. This is most likely to occur when a crystal size of about 50.mu., or larger, is reached in any dimension. Plugging of spray nozzles is a significant economic detriment for the reasons that (1) manpower time must be expended to discontinue spraying operations and remove the plugging material and (2) any area sprayed before the plugging is detected and corrected is likely to have been covered improperly. Thus, there is a considerable practical need for a means of controlling metribuzin crystal growth in these spraytank combinations.